Filters with colored stripes are used in certain color television cameras. The filters are generally small, i.e., less than 25 mm diameter and the stripes closely spaced, in the order of 10 to 40 stripes per millimeter. The colored stripes are typically produced as thin film dichroic coatings, that is coatings that transmit one part of the visible spectral range and reflect the rest. Two or three sets of colored stripes of different colors may be used on a filter. The different sets of stripes may be either parallel interspersed or may be crossing each other at some given angle. Cross striped dichroic filters are described in U.S. Pat. No. 3,771,857 and interspersed parallel striped filters are described in pending application Ser. No. 305,692, filed on Nov. 13, 1972. It has been found that such striped filters must be made with a high degree of accuracy to perform satisfactorily in a color television camera. The stripes must be very accurate and uniform in geometry and colorimetry and virtually free from flaws to produce perfect television pictures. The striped filter is placed right at an image plane in the color television camera, but the image created by the stripe pattern is electronically removed from the signal so that the stripes themselves will not show on the television screen. Any irregularities and flaws in the stripe pattern will, however, clearly show up on the screen and are, therefore, very objectionable.
The striped filters are frequently built right into the image tube, normally of the vidicon type, in the color television camera. It is, therefore, necessary to inspect the filters carefully before committing them to become an integral part of expensive image tubes. It has been found to be very difficult and time consuming to adequately inspect the striped filters by any conventional optical methods such as microscopy and magnifying by projection because the very dominant visual image of the colored stripes tends to obscure any flaws in the pattern and because the allowable irregularities in geometry and colorimetry are too small to readily be observed by visual means.